Thermostat in COOL - Fan works - Compressor does not

Place thermostat in the OFF position. Locate the HIGH PRESSURE switch mounted on the top of the air conditioner unit. On units more than three years old, check if metal tab on the RESET button is extended. If tab is extended push it in. On units less that three years old, push the red tab on the green switch with the RESET sticker located on the body of the device. These switches shut power off to the compressor (and sea water pump on three year or newer units) when cooling water is insufficient to make the required heat exchange as described in Theory Of Operation in your manual. Check water pump for prime and motor operation, check raw water strainer for clogging and hoses for kinks. Fix water problem and turn thermostat back on. 

If water pump is operational and HIGH PRESSURE switch has not activated, perform thermostat jumper test described in your manual.  If you do not have your manual, remove the faceplate of the thermostat exposing the Red, Green, Black and White wires.  You will notice there is also a Red jumper wire in place.  Be sure your circuit breaker is in the OFF position at this time.  Once the breaker is OFF, remove the single Red wire, the single Black wire, and the single Green wire.  Leave the Red jumper wire in place.  Manually twist together the Red, Green, and Black wires.  Once completed, turn the units circuit breaker back to the ON position.  The entire system should engage in the COOL mode at this time.   If unit operates, replace thermostat batteries and/or thermostat.